1. Field of the Invention
The present invention relates to mobile communication, and especially to a method for supporting a relocation of an IP session during a network layer handover in a mobile communication system, and a mobile node and a network node supporting the method.
2. Description of the Related Art
A mobile communications system refers generally to any telecommunications system wherein the access point to the system may change when users move within the service area of the system. The mobile communications network is, correspondingly, an access network providing an end user with wireless access to external networks, hosts, or services offered by specific service providers. The service area of the system may comprise different access technologies and several administrative domains.
The new mobile communication systems have been developed to facilitate widespread use of new applications, also including ones that require more bandwidth and extended transmission sessions compared to earlier technologies. On the other hand, the ubiquitous coverage of current cellular systems has led the end users to expect similar availability of services from the next generations of systems. Therefore, seamless service provisioning for the considerable range of different applications will be a critical issue for the success of the new mobile communication systems.
In the context of providing wireless access using the Internet Protocol (IP), seamless IP layer mobility refers to the ability to hand over a mobile node (MN) to a new access router (AR) with minimal disruption to the IP connectivity. Under the auspices of the Internet Engineering Task Force (IETF), a number of solutions for seamless IP layer mobility have been generated. Mobile IP, as defined in Request for Comments (RFC) 2002, is an enhancement of the Internet Protocol version 4 (IPv4) that adds mechanisms for forwarding Internet traffic to mobile nodes when they are connecting through a network other than their home network. Similar mechanisms have been developed for Internet protocol version 6, referred to as IPv6. Each mobile node is assigned a permanent home address on its home network and a care-of address that identifies the current location of the device within a network and its subnets. Each time a mobile node moves to a different network, it acquires a new care-of address. A mobility agent (also known as Home Agent) on the home network associates each permanent address with its care-of address.
As an enhancement to this, fast handover protocol allows a mobile node to configure a new care-of-address before it moves towards a new subnetwork with the aim of being able to use it directly after its connection to the new access router. Consequently, the latency time is minimized and potential loss of packets during handoff is effectively eliminated.
In the process of establishing the new forwarding path for IP flows, a mere creation of connection to the new nodes, however, might not be enough. The nodes along the new path must be prepared to provide similar forwarding treatment to the IP packets. This is especially important for services with particular requirements, such as time sensitive VoIP telephony and video and streaming services, whose successful employment in mobile environment depends heavily upon the ability to minimize the impact of the traffic redirections. A context transfer procedure is a specified method, which aims at provisioning of seamless IP layer connectivity. Context relates to the information transferred from one network entity to another as a means of re-establishing routing related services on a new subnet or a group of subnets. Context transfer thus facilitates seamless transfer of the mobile node's (also known as mobile terminal, station or device) packet session to a new access router while the session can be re-established without having to perform the entire protocol exchange between the new node and the mobile node.
In order to perform fast handover and context transfer procedures as described above, the Candidate Access Router Discovery (CARD) provides means for discovering the IP addresses of the candidate access routers, and such characteristics of the access routers that may be of interest to an MN when the access router is evaluated as a handover candidate. Through this candidate access router discovery (CARD), at the time of the IP layer handoff, the candidate access router whose capabilities appropriately match with the current requirements of the mobile node may be selected as a target access router.
The IETF Seamoby Working Group Internet Drafts “Candidate Access Router Discovery” of October 2002 and “A Dynamic Protocol for Candidate Access-Router Discovery” of October 2002 (work in progress) define two approaches for a protocol that discovers the identities of candidate access routers. However, neither of these drafts specifically define the content nor format the capability information of each access router. In the description of the procedures, it is assumed that the entire capability information is always exchanged between the communicating entities. This, however, introduces in some disadvantages to the information transfer.
Firstly, it is not likely that all data in the capability information is relevant to the decisions of IP layer handoff. On the other hand, the relevance of individual parameters of the capability information depends on the dynamically changing requirements of the sessions. For example, if the essential criteria for deciding on a next access router is the capability to provide an appropriately wide bandwidth, most of the other data in the capability information of a particular access router, such as port information, supported application functionality, authentication settings etc., is irrelevant to the actual choice. On the other hand, if the essential issue for the seamless service is the existence of a certain transcoding functionality in the new access router, the information on available QoS parameters may be redundant at that point of time. Furthermore, the unnecessary transmission of irrelevant data is especially disadvantageous when the capability information is to be transferred between a mobile node and one or more network nodes, utilizing the already scarce radio resource. The term essential in this context thus relates to a capability that facilitates maintaining an ongoing IP session during relocation, typically a special feature that is not by default available in the access router. The information on whether the essential capability is available in the access router or not is applicable for choosing the next access router for relocation of an IP session.
Additionally, there can be an inclination to minimize the amount of information exchanged between parties without any established confidentiality relationship, for example between elements under different administrative control. Rather than declaring as much as capability information and application requirements, along with their particular values, the mobile nodes and access routers may tend to reveal information only on a need-to-know basis.